The present invention relates to nickel coated copper foils which are readily etched. More particularly, it concerns a printed circuit board or blank which comprises a resinous substrate having bonded thereto, via a barrier layer of nickel, a layer of copper foil with the layer of nickel being characterized by its unique etchability.
Printed circuits are widely used in a variety of electronic applications, such as radios, televisions, computers, etc. Of particular interest are multi-layer laminates which have been developed to meet the demand for miniature electronic articles and the growing need for printed circuit boards having a high density of interconnections. These laminates of synthetic plastics or resins and copper foil are made in such a way that circuits are possible not only on the surface but also spaced throughout the thickness of the laminates. In order for the single or multi-layer laminate to operate satisfactorily the resistivity of the plastic layer and the peel strength of the copper foil, among other things, must be maintained as high as possible. Thus, strict production quality control measures are followed, and special requirements on raw materials, such as the copper foil and the adhesive, are imposed. In U.S. Pat. No. 3,220,897 there is disclosed a copper foil which has been treated electrolytically to provide it with a "nodularized" surface for better adhesion. Similarly, in U.S. Pat. No. 3,293,109 a copper foil is found to have better adhesive properties when provided with an external surface having myriad minute projections whose inner cores contain copper-copper oxide particles, the minute projections being encapsulated by a copper coating.
The two types of copper foil as taught by the two above-mentioned patents are excellent when it comes to adhesion, whether in one layer or multi-layer laminates. One source of difficulty, however, has been the frequent appearance of stains and spottings throughout the resinous layer of the finished printed circuit boards. These stains of which brown spotting is a particularly troublesome type tend to adversely affect the dielectric properties of the resin and consequently the over-all performance of the printed circuit. Likewise, the physical appearance of the final product is undesirable.
The actual mechanism for this staining is not fully understood; however, the cause appears to be the result of chemical and/or mechanical interactions between the copper foil and the resin layer. The lamination step which involves high pressure-high temperature treatment seems to give rise to such interactions which are manifested as degradation of the adhesion of the foil upon heat aging together with staining of the epoxy/glass board.
To overcome the above-discussed staining and spotting problems, a technique has been developed wherein a barrier layer of metal is applied to the surface of the copper foil which is to be bonded to the resinous substrate. This system is described in U.S. Pat. No. 3,585,010. In practice, the printed circuit is formed by selectively etching away portions of the copper foil and nickel barrier layer. While this system has met with success, when nickel is employed as the barrier layer it suffers from the inherent limitations that of the common copper etchants only ferric chloride also readily dissolves nickel. Unfortunately, in many commercial operations it is often desirable to use copper etchants other than ferric chloride.
Accordingly, it is the principal object of the present invention to provide a nickel barrier layer which is readily etchable by a plurality of common copper etchants.
Other objects of the invention will become apparent to those skilled in the art from the detailed reading of the specification and claims.